There is known a vehicle shock absorber capable of exerting a variable damping force. The shock absorber includes, for example, a cylinder, a rod-side chamber and a piston-side chamber partitioned by a piston inside the cylinder, a reservoir, a main passage and a relief passage that causes the rod-side chamber to communicate with the reservoir, an expansion-side passage that causes the reservoir to communicate with the piston-side chamber, a contraction-side passage that causes the piston-side chamber to communicate with the rod-side chamber, a back-pressure chamber provided in the middle of the relief passage, a pressure control solenoid valve provided in the middle of the relief passage to control a pressure of the back-pressure chamber, a relief orifice provided in the middle of the relief passage in a downstream side of the pressure control solenoid valve, and a damping force control valve provided in the middle of the main passage, biased to a valve closing direction by virtue of the pressure of the back-pressure chamber as a pilot pressure, and biased to a valve opening direction by virtue of an upstream-side pressure (see JP 2009-250396 A).
The shock absorber is also provided with check valves in the expansion-side passage and the contraction-side passage in order to circulate hydraulic oil in the order of the reservoir, the piston-side chamber, the rod-side chamber, and the reservoir in a one-way manner.
Therefore, as the shock absorber makes an expanding or contracting motion, the hydraulic oil is extracted from the rod-side chamber and is discharged to the reservoir through the damping force control valve provided in the main passage.
If the pilot pressure for biasing the damping force control valve to a valve closing direction is controlled by using the pressure control solenoid valve, the valve opening pressure of the damping force control valve is controlled. As a result, the damping force generated by the shock absorber is controlled.
In the shock absorber described above, the relief orifice is provided in the downstream side of the pressure control solenoid valve so as to suppress a flow rate change of the hydraulic oil passing through the relief passage. Therefore, it is possible to suppress the pressure control solenoid valve from generating a vibration by repeating opening and closing operations when the pressure of the upstream side of the pressure control solenoid valve oscillates in the vicinity of the valve opening pressure. As a result, the damping force exerted by the shock absorber is stabilized without oscillation.